1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine or a printer using the electrophotographic method or the electrostatic recording method.
2. Related Background Art
FIG. 7 of the accompanying drawings shows a cross-sectional view of an image forming apparatus which is the background of the present invention.
The reference numeral 10 designates a photosensitive member, the reference numeral 13 denotes developing means, the reference numeral 16 designates transfer charging means, and the reference character 19a denotes a charge eliminating needle as a charge eliminating member.
An electrostatic image formed on the photosensitive member 10 is visualized into a toner image by the developing means 13. In order to transfer this toner image to a transfer material, use is made of transfer charging means for imparting charges (back charges) to the back of the transfer material in a transfer area, and electrostatically attracting the toner image to the transfer material.
In recent years, as transfer chargers, contact transfer chargers such as transfer rollers having such merits as the compactness of power source capacity and a small amount of production of discharge product typified by ozone, as compared with a well known corona charger or the like have spread, and such chargers of various materials, sizes and volume resistivities have been practically used.
The corona charging transfer method imparts back charges to the back of a transfer material by discharge into the air and therefore, the outputted current value generally need be about several mA, whereas in the contact transfer charging method, the outputted current can be suppressed by about a figure to about three figures, i.e., to about several .mu.A to about several hundreds of .mu.A. However, of such contact transfer means, a more accurate control method is required in order to make the charges directly imparted to the back of the transfer material as uniform as possible, and to adapt the charges to changes in the transfer condition and the environment.
So, there is also known a transfer voltage controlling method, called ATVC control, of constant-current-controlling the transfer charging means by a predetermined current value during the non-supply of sheets, and constant-voltage-controlling the transfer charging means by a voltage value determined on the basis of the voltage at this time during the supply of sheets, in order to apply a proper transfer voltage irrespective of the size of the transfer material, and the atmosphere and environment such as temperature and humidity (Japanese Patent Laid-Open Application No. 2-123385).
The transfer material after transfer is liable to twine around the photosensitive member 10 because it is charged by the transfer charging means 16, but the electrostatic attraction between the transfer material and the photosensitive member 10 is weakened by the charge eliminating needle 19a as the charge eliminating member as previously described, to thereby make the separation of the transfer material from the photosensitive member easy.
However, the transfer material passing above the charge eliminating needle is not always constant in its distance from the charge eliminating needle 19a over its entire area and therefore, even if in the transfer area, the back of the transfer material is uniformly charged, irregularity occurs to the amount of back charges residual on the back of the transfer material after the transfer material has passed above the charge eliminating needle 19a.
A little more specifically speaking, the leading end portion of the transfer material with respect to the direction of conveyance thereof (hereinafter the leading end and the trailing end will all be referred to as so with respect to the direction of conveyance) is conveyed with the rotation of the photosensitive member 10 while being attracted to the photosensitive member 10 by the electrostatic attraction, and the transfer material has its charges eliminated near the charge eliminating needle 19a and is separated from the photosensitive member and therefore, as indicated at 18b in FIG. 7 the transfer material passes a route somewhat near to the photosensitive member 10 by the time when it arrives at a conveying member 20 from the transfer area.
However, when a certain degree of range of the transfer material is separated, the transfer material is affected by the gravity of the leading end thereof and the rigidity (stiffness) of the transfer material itself and therefore, the trailing end thereof becomes liable to separate as compared with the leading end, and the transfer material tries to pass a route near to a portion 18a in FIG. 7.
Further, the charge eliminating needle 19a is usually constructed with a protective member 19b (or a guide member serving also to prevent contact) to prevent the transfer material from contacting the charge eliminating needle, and when that portion of the conveying member 20 which supports the transfer material and is nearest to the charge eliminating member 19a is constructed at a location higher than the upper end of the protective member 19b, the trailing end portion of the transfer material depends from the conveying member 20 by the gravity thereof after it has passed the transfer area, and becomes nearer to the charge eliminating needle 19a.
Usually the charge eliminating needle 19a is at ground potential or has applied thereto a voltage of the opposite polarity to the back charges and therefore, particularly when such a voltage is applied thereto, the nearer to the charge eliminating needle 19a is the transfer material, the more strongly affected by the electric field of the charge eliminating needle 19a is the transfer material, and more of the back charges are eliminated.
When, in such a case, the transfer charging means 16 uniformly imparts charges to the whole area of the transfer material, unevenness (irregularity) occurs to the back charges residual on the back of the transfer material after the elimination of the charges and particularly, the back charges near the trailing end portion become deficient. Thereupon, the electrostatic toner holding force of the transfer material becomes weak and the toner becomes liable to scatter or the electrostatic attraction between the trailing end portion of the transfer material and the conveying member weakens, and the disturbance or the like of the image by the abnormal behavior of the trailing end portion (hereinafter referred to as the trailing end jump) in a fixing portion becomes liable to occur.
On the other hand, when the transfer output is set to a rather high level from the leading end side in back charges become less and rather many charges are uniformly imparted to the transfer material, the back charges become excessive in the leading end portion to the central portion, and the charges go through the transfer material and negate the charging charges of the toner in the toner image. Particularly in the case of an image forming apparatus of the so-called reversal developing type in which the charging polarity of an image bearing member and the polarity of charges imparted by transfer charging means are opposite to each other, an excessive electric current flows between the transfer charging means 16 and the photosensitive member 10 immediately before the leading end portion of the transfer material comes to the transfer area, and that portion on the photosensitive member 10 through which the excessive electric current has flowed is not well charge-eliminated even if it is subjected to pre-exposure means 24. Therefore, a portion which cannot be sufficiently charged by a primary charger 11, i.e., a so-called charging memory, is created on the photosensitive member 10, and a poor image appears.